Normal body temperature varies over the course of the day, controlled in the thermoregulatory center located in the anterior hypothalamus. The body is normally able to maintain a fairly steady temperature because the hypothalamic thermoregulatory center balances the excess heat production, derived from metabolic activity in muscle and the liver, with heat dissipation from the skin and lungs. However, faced with environmental extremes, humans cannot maintain the narrow daily variation of body temperature without the aid of clothing and protective environments [1].

In 1992, a detailed study of the range of oral temperature readings in 148 healthy men and women aged 18 to 40 was reported using over 700 measurements [3]. Oral temperatures in the cohort ranged from 35.6°C (96.0°F) to 38.2°C (100.8°F) with a mean of 36.8 ± 0.4°C (98.2 ± 0.7°F). Low levels occurred at 6 AM and higher levels at 4 to 6 PM. The maximum normal oral temperature at 6 AM is 37.2°C (98.9°F), and the maximum level at 4 PM is 37.7°C (99.9°F), both values defining the 99th percentile for healthy subjects. From these studies, a morning reading >37.2°C (98.9°F) or an afternoon temperature of >37.7°C (99.9°F) would be considered a fever. Rectal temperatures are generally 0.6°C (1.0°F) higher than oral readings. Oral readings are lower probably because of mouth breathing, which is particularly important in patients with respiratory infections and rapid breathing. Temperature measurements from the lower esophagus reflect core temperature, and tympanic membrane temperature readings are also close to core temperature.

The normal daily temperature variation is typically 0.5°C (0.9°F). However, in some individuals recovering from a febrile illness, this daily variation can be as high as 1.0°C. During a febrile illness, daily low and high temperature readings are maintained but at higher levels.

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